Luminance contrast ratio, besides other measurements, is used to characterize the viewing-angle of a liquid crystal display. The luminance contrast ratio (CR) is defined, for a normally black display as, the ratio of the on-state transmitted luminance (I.sub.on) to that of the off-state (I.sub.off), as follows: EQU CR=I.sub.on /I.sub.off.
The operational principle of a TN LCD display is based on the electric field controllable birefringence of the liquid crystal layer. The polarization state of a light beam after passing through the liquid crystal layer depends on the length of the optical path, the twist angle and the birefringence of the nematic liquid crystal. In a normally black TN LCD display the cell gap is normally optimized such that when the cell is in the off-state, at normal incidence, the polarization state of the light passed through the 90.degree. twisted liquid crystal layer is linear and perpendicular to the transmission axis of the exit polarizer. This gives a low I.sub.off and therefore a high contrast ratio. When the inclination angle (which is measured of of normal from the display surface) increases, the optical path increases and the effective birefringence of the liquid crystal layer changes. Consequently, the polarization state of the light passed through the cell becomes more and more elliptical as the inclination angle increases. Therefore, transmission increases and the contrast ratio decreases.
It has been discovered in the prior art that retardation films may be used to partially compensate for the change in effective birefringence of the liquid crystal layer as a result of increasing inclination angle, such that within a larger inclination angle the linearly polarized light beam passing through the retardation films and the cell remains fairly linearly polarized. The following prior art publications relate to the use of retardation films in LCDs: (1) Yamagishi, N., Watanabe H., Yokoyama, K., "Wide Viewing Angle LCD Using Retardation Film" Japan Display 189, page 316; and (2) Nagatsuka, Tatsuki, and Yoshimi, Hiroyuki, "Retardation Films for LCD" technical paper released in 1989 by Nitto Denko corporation, Tokyo, Japan.
The use of retardation films in liquid crystal displays developed from research in super-twisted nematic (STN) displays. Because of the large twist angle, STN displays exhibit strong yellow or blue coloration. The use of retardation films provides an economical and lightweight solution for removing such coloration.
In addition to the use of retardation films, researchers in liquid crystal technology have examined a number of other alternatives to broaden the viewing angle of LCDs. These methods include the use of tilted homeotropic alignment, the use of double cells, and the use of two-domain alignment.
Dichroic dyes have also been widely used in liquid crystal displays. The majority of dichroic displays operate using a phase change mode with contrast ratio of approximately 25:1 in reflective mode and approximately 5:1 in transmissive mode. A discussion of the use of dichroic dyes in liquid crystal display technology is provided in the following publications: Bahadur, Birendra, "Current Status of Dichroic Liquid Crystal Displays", Mol. Cryst. Liq. Cryst., 1991, volume 209; and Bahadur Birendra "Dichroic LCDs" in the book "Liquid Crystals--Applications and Uses", volume III, B. Bahadur editor, published by World Scientific, Singapore (1992). Unfortunately, research in the application of dichroic dyes to LCDs has been somewhat curtailed by a general mis-apprehension of LCD researchers concerning the photo and chemical stability of such dyes and the related problems.